User interface guidelines from stan wiki

[bob-carpenter]

converting to PR for community feedback; originally from stan-dev/stan wiki page:

https://github.com/stan-dev/stan/wiki/User-Interface-Guidelines-for-Developers/

Last edited by @riddell-stan.

[riddell-stan]

I'm willing to work on finalizing this.

I'm thinking of starting by removing all mention of unresolved items. There's plenty of resolved material to include.

@bgoodri Any changes in y'all's thinking about RStan 3?

[riddell-stan]

My plan is to update this with a note saying that PyStan 3 is going to be released in 2021 using an API described in this document. If RStan 3 emerges and introduces significant changes to the API described here, a PyStan 4 release can be made which will align with the RStan interface.

[riddell-stan]

I would like to work on this. I don't have permission to add commits here. What would be the best way forward?

There's only one small change I'd like to make: add a note saying that this document is being preserved here and mentioning that PyStan 3 was released in March 2020.

cc @bbbales2 @rok-cesnovar as you two seem to be active on this repo.

[rok-cesnovar]

@riddell-stan you should be able push directly now.

[riddell-stan]

I think this is ready for review. The goal here is just to preserve the document. It reflects several hours of in-person discussions in NYC. /cc @bob-carpenter @bgoodri